Dehumidification of air in HVAC systems typically takes place through the use of the evaporator in cooling mode. One drawback to using an evaporator, alone, for dehumidification, is the excess reduction in air temperature that results, which is commonly referred to as overcooling. Overcooling occurs when air that is subject to dehumidification is cooled to a temperature that is below the desired temperature of the air. Overcooling is a particular problem when the dehumidification is required in a room that is already relatively cool. Overcooling generally involves air temperatures of approximately 50° F. to 55° F. or lower.
Overcooling has been addressed by utilization of a reheat coil, as disclosed in U.S. Pat. No. 5,752,389 (the '389 Patent). Air that is overcooled by the evaporator is passed over the reheat coil in order to increase the temperature of the overcooled, dehumidified air to a desired temperature. In the '389 Patent, the reheat coil is heated by diverting hot refrigerant gas through the reheat coil when dehumidification is required. Reheat may also be provided by alternate heat sources, such as electric heat or gas heat. The reheat coil system for providing heat to the dehumidified, overcooled air has several drawbacks including the requirement of additional equipment and/or piping and/or additional energy input. The presence of an additional coil in the indoor air stream results in losses that must be overcome by the indoor blower. These losses are present any time the indoor blower is running, regardless of the operational mode of the unit. The result is higher relative energy usage to circulate air with an additional coil present.
Another dehumidification method known in the art is disclosed in U.S. Pat. No. 4,182,133 (the '133 Patent). The '133 Patent is directed to a dehumidification method that controls refrigerant flow through circuits within the indoor coil of an air conditioning/heat pump unit. The '133 Patent system, when providing dehumidification, has a liquid header that distributes the refrigerant across several circuits within the indoor coil. At the opposite end of the indoor coil, the outlets of the various circuits of the coil are allowed to flow into a single common vapor header. The liquid header at the inlet of the indoor coil contains a solenoid valve that may be closed to prevent refrigerant flow to one or more of the circuits within the coil. The '133 Patent system operates such that when humidity reaches a certain level, the valve in the liquid header is closed in order to limit the number of available circuits for refrigerant flow. The area of the indoor coil that remains in the active circuit and receives refrigerant flow, experiences an increase in refrigerant flow through a given heat transfer area. The increased flow of refrigerant results in a greater amount of moisture being removed from the air in that portion of the indoor coil. The distribution to the parts of the indoor coil is achieved through a single liquid header. The operation of the '133 Patent system is only concerned with removal of humidity. One drawback of the '133 system is that the dehumidified air is not reheated and may be overcooled. Another drawback of the '133 system is that the inlet header does not distribute flow across the circuits of the evaporator, leading to uneven phase distribution of refrigerant across the evaporator heat exchanger.
Therefore, what is needed is a method and system for dehumidification that dehumidifies air without overcooling and provides a system that can be retrofitted into existing systems.